1. Field of The Invention
This invention relates to a semiconductive device and its fabrication and more particularly, to a MOS semiconductive device having a good withstanding voltage characteristic of the gate insulating film and a method for making such a device as mentioned above.
2. Description of The Prior Art
In order to realize high performance of MOS semiconductive devices, it is a recent trend to miniaturize the semiconductive device by utilizing the scaling rule. However, the miniaturized semiconductive devices involve the following problems.
(1) Since the depth of the junction of source/drain becomes small, the parasitic resistance of the source/drain increases. Although the shortage of the channel length makes it possible to increase the drain current, it is difficult to attain a drain current which will be passed through the MOS transistor prior to the miniaturization.
(2) Since the gate insulating film is formed as thin, the withstanding voltage characteristic of the gate insulating film is lessened. In addition, a phenomenon is observed wherein the gate current increases by tunneling. This tunneling phenomenon becomes more pronounced when the gate oxide film is made of a silicon oxide film with a thickness of not larger than about 5 nm.
In order to solve these problems, several proposals have been made as follows. With respect to the problem (1), for example, the source/drain are converted into silicide on the surfaces thereof. With respect to problem (2), a silicon nitride film which has a specific inductive capacity higher than the silicon oxide film is formed as the gate insulating film.
However, these proposals have also the following problems.
The specific inductive capacity of silicon nitride film, which is 7.4, is not much higher than that of silicon oxide film which is 3.4. Accordingly, the silicon nitride film is not considered as useful for solving the problem (2). Thus, the above. problem (2) has not been satisfactorily solved yet. In addition, the proposed technique of silicificating the surface portions of the source/drain presents another problem in that the fabrication process undesirably increases in number of steps.